1. Field of the Invention
The present invention relates to a field effect transistor employing a semiconductor film composed of an oxide as the channel. The present invention relates also to a process for producing the transistor.
2. Description of the Related Art
The field effect transistor (FFT) is a three-terminal element comprising a gate electrode, a source electrode, and a drain electrode. This FET is an electron-active element for controlling and switching, by application of a voltage to a gate electrode, the current flowing through the channel layer between the source electrode and the drain electrode. In particular, the FET employing a semiconductor thin film as a channel layer is called a thin film transistor (TFT).
The TFT, based on the thin film technique, can readily be formed in a large area on a substrate advantageously. Therefore, the TFTs are widely used as a driving element of a flat panel display like a liquid crystal display. In the liquid crystal display (LCD), the pixels are turned on and off by TFT formed on a glass substrate. For prospective organic LED display (organic light-emitting diode display, OLED) of a high performance, current driving of a pixel by TFT is considered to be effective.
The TFTs employing a polycrystalline silicon film or an amorphous silicon film are widely used as the material for the channel layer at the moment. However, the amorphous silicon TFT or the polysilicon TFT is necessarily processed at a high temperature in the device production, which makes difficult the formation of the thin film on a substrate like a plastic substrate or a film.
Lately, semiconductor materials composed mainly of an oxide are attracting attention as the material for the TFT to be formed on a polymer substrate, a film, or a glass substrate.
For example, TFTs are developed actively which employ a transparent oxide thin film constituted mainly of ZnO (zinc oxide). Further, a thin film transistor is disclosed which employs an In—Ga—Zn—O type of amorphous oxide. Such TFTs are described in documents, for example: U.S. Patent Application Publication No. 2007/0194379 (corresponding to International Publication No. 2005/088726), and Nature, vol. 432, pp. 488-492, (2004). Such a transistor can be formed at a room temperature on a plastic or glass substrate, and has a transistor characteristic of normally-off type at an electric field effect mobility of about 6-9 cm2(Vs)−1.
For satisfactory characteristics of the transistor, the resistance between the semiconductor and the source electrode or the drain electrode is preferably lower, and the electric connection is preferably stabler in the environment or in the driving. For this purpose, electrodes of the amorphous oxide thin film transistor are disclosed which employ oxide electrodes such as ITO (In2O3:Sn, tin-containing indium oxide), and metal electrodes such as Au (gold) (Thin Solid Films, 516 (2008) 5899).
Various metal electrodes are described for electric connection to a crystalline ZnO semiconductor in a document: Journal of Crystal Growth. 287 (2006) 149.
As the source electrode and the drain electrode of the TFT employing an oxide semiconductor, several kinds of electrodes are described in the above-cited document (Thin Solid Films 516 (2008) 5899). However, further improvements in the electric contact and in the reliability are necessary. A noble metal such as gold does not have sufficient adhesiveness to an oxide semiconductor, and is less reliable in that the metal electrode produced by a productive process like vacuum deposition or sputtering is easy to be peeled off. In contrast, an electrode composed of an oxide like In2O3:Sn (ITO) is preferred for the adhesiveness. However, the surface property of such an oxide electrode can be affected in the film forming process or the post-treatment including a heat treatment and a solution treatment to cause variation in electric connection with the semiconductor.
An object of the present invention is to provide an electrode which enables stable electric connection to the oxide semiconductor with a lower contact resistance.
On the other hand, amorphous InGaZnO4 has an optical band gap of about 2.9 eV, being transparent to visible light. However, a transistor employing the amorphous InGaZnO4 as the channel can change its characteristic on exposure to near-ultraviolet light having a wavelength of shorter than about 450 nm (higher than about 2.7 eV). The transistor using such a material vary their characteristics. Display device using the transistor tends to cause deterioration of the image quality on irradiation of a display light or an external light onto the TFT.
To prevent the image quality deterioration, a light-shielding layer may be placed on or under the TFT to intercept the light. This will limit the layout of the TFT and increase the production steps. Further, even with the light-shielding layer, depending on the constitution, a trace amount of stray light can irradiate the TFT, unexpectedly. (The stray light herein signifies the light introduced to the TFT from an unspecified portion by reflection or refraction by inside or outside of the element.) Therefore, the TFT insensitive to the light is desired.
A second object of the present invention is to provide a thin film transistor which has characteristics less affected by irradiation of stray light.